Biomimetic prosthetic device

ABSTRACT

A prosthetic device includes a phalanges portion, a metatarsals portion that is movably coupled to the phalanges portion, an ankle portion that is movably coupled to the metatarsals portion, and a calcaneus portion that is movably coupled to the ankle portion.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a non-provisional of and claims the benefit of U.S.Provisional Patent Application No. 62/516,333, filed on Jun. 7, 2017,the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The human ankle is crucial to mobility as it counteracts the forces andmoments created during walking. Currently there are nearly 2 millionpeople living with limb loss in the United States. Many of theseindividuals are either transtibial (below knee) or transfemoral (aboveknee) amputees and require an ankle-foot prosthesis for basic mobility.While there are an abundance of options available for individuals whorequire an ankle-foot prosthesis, these options fail to mimic an intactankle when it comes to key evaluation criteria such as range of motion,push-off force, and roll over shape.

The simplest type of ankle-foot prosthesis is the conventionalnon-articulating SACH (Solid Ankle Cushioned Heel) foot shown in FIG. 6.The SACH foot is able to closely resemble the shape of an actual footand provides the user with some cushioning during movement. However, itis unable to provide the range of motion and energy return of an intactankle. Regardless, many less active amputees prefer the SACH footbecause of the greater control it gives the amputee.

Unlike the SACH foot, the dynamic response ankle-foot shown in FIG. 11stores energy during the beginning of the gait cycle and uses the storedenergy to propel the foot forward. Also called ESR for Energy Storingand Returning, the energy storage mechanism of the dynamic responseankle-foot is similar to the role of the Achilles tendon. During gait,the Achilles tendon is stretched and stores potential energy that isreleased during push-off. The energy storage mechanism in dynamicresponse feet are typically primarily weight activated. This means theprosthetic will store energy while the individual is standing unlike anable-bodied ankle. Despite this difference, the dynamic responseankle-foot provides some resistance to movement similar to that of anintact ankle.

Dynamic response feet can be further classified as either passive oractive (microprocessors). Because the energy produced by the ankle jointduring average walking speeds is almost completely self-sustaining withno net external energy loss, there is the potential for a purelymechanical mechanism such as the dynamic response ankle to generate theforward motion necessary for an able-bodied gait. However, for speedsfaster than normal walking, passive systems are not capable of fullyemulating an intact ankle because a positive net external energy isproduced by the ankle. The use of an active ankle foot prosthesis forfaster speeds may be necessary in the future, but current designlimitations make this application less than ideal. An active ankle-footprosthesis can be over twice as heavy as a conventional prosthesis, areexpensive, and experience hardware and control issues adjusting todifferent speeds. Fundamentally, active prosthetic ankle-feet operateusing preplanned kinematic trajectories as opposed to the impedancecontrol mechanism of a human ankle. Finally, while still operating as anESR system, active ankle-foot prostheses are difficult to customize ormatch biomimetically in size and weight.

The amount of energy stored in the prosthesis is dependent on thestiffness. Increasing the stiffness will increase the propulsion forces,however, it simultaneously decreases the range of motion (ROM) of theankle. The ankle joint has a ROM from about 45° plantar flexion to 20°dorsiflexion. Forced to make a choice between propulsion forces andrange of motion, many ankle-foot prostheses have only been designed forthe ROM that is experienced during gait on an even surface, a value ofno more than 30°. While this may seem sufficient as the ROM of the ankleremains consistent with changes in speed, a study looking at individualswith limited ankle ROM due to a sprain showed that ankle ROM does impactgait symmetry in regards to step length and step time. Additionally,ankle ROM is important for walking on sloped surfaces as it helpsaccommodate for movement about different equilibrium positions.

While both the kinematics and kinetics of an intact ankle are importantto its functionality, so far it has been impossible for a passiveprosthetic ankle-foot to mimic both. There exists a discrepancy betweendesign changes that improve the kinematics and kinetics. The effect ofincreasing stiffness is an example of this discrepancy. In anable-bodied ankle, the relationship between angle and push-off moment islinear. However, most prostheses are built with a stiff plastic boardthat resembles a cantilever beam. A rudimentary knowledge of cantileverbeams tells us that the linear relationship between deflection and forceis restricted to small deflections and much less than the ankle angleexperienced by an able-bodied individual. The stiffness of the foot alsoimpacts the location of the ground reaction forces, and therefore therollover shape as discussed below. Olesnavage and Winter noticed thiseffect and suggested the use of a rigid constraint to prevent the footfrom over-deflecting.

Recent research in active prostheses has been able to demonstrate theeffectiveness of applying a torque that is linear with ankle angle insingle subject experiments in a lab environment. Caputo and Collins useda Universal Ankle-Foot Prosthesis Emulator that determined the desiredtorque by a piecewise linear function in 2014. A team at the Roboticsand Multibody Mechanics Research Group at the Vrije Universiteit Brusselis making progress in mimicking both kinematics and kinetics in thedevelopment of the actuated prosthetic AMP-Foot. Although not explicitlystated, one of the major changes between the AMP-Foot 2.0 tested in 2014and the AMP-Foot 3.0 in 2016 was a linear relationship between torqueand ankle during initial contact to flat foot. The change resulted in acurve that better mimics an intact ankle as provided by Winter's dataand an extra 5 Joules of energy storage. It is interesting to note thatthe strategy used in the design of active prosthetics to achieve bothpush-off and range of motion in fast walking speeds is to effectivelyincrease stiffness with ankle angle. While this strategy has beenapplied to the design a quasi-passive prosthetic ankle-foot thatincreases the stiffness with ankle angle using a cam-based transmissionand an active sliding support beneath the leaf spring, the strategycannot be used in a completely passive prosthesis because it requirespositive work to be done by the prosthetic, nor should it be necessaryfor normal walking speeds.

Hansen developed a characteristic of gait called the roll over shapethat incorporates both the kinematics and kinetics. The roll over shapeis created by plotting the center of pressure during a step in ashank-based coordinate system. Recent research, summarized by Hansen andChildress, has found that “roll-over shapes in able-bodied subjects donot change appreciably for conditions of level ground walking, includingwalking at different speeds, while carrying different amounts of weight,while wearing shoes of different heel heights, or when wearing shoeswith different rocker radii”. This suggests that able-bodied individualswill alter their ankle kinematics to preserve their roll-over shape.However, amputees do not have the adaptive control that an able-bodiedindividual has over their roll-over shape. Therefore, the design of theprosthetic predominantly controls the roll-over shape an amputee willproduce. As a result, it has become a method to evaluate prosthetics.However, while the roll over shape demonstrates the relationship betweenkinematics and kinetics, it is not directly impacted by magnitude. Otherevaluation methods are necessary to determine the late stance push-off.

SUMMARY OF THE INVENTION

Human gait has evolved to maximize energy efficiency through adaptationssuch as beginning the gait cycle with heel strike. The role of the anklejoint is crucial to healthy and efficient gait. As a result, individualslacking an ankle consume over 20% more oxygen than able-bodiedindividuals. The prospect of human augmentation has motivated prostheticdesigns that sacrifice characteristics of the evolved able-bodied anklefor enhanced functionality in a specific area. However, for unilateralamputees in particular, any deviation in functionality from theable-bodied ankle causes gait asymmetry and requires extra effort by theamputee to compensate. By taking a more biomimetic approach to theprosthetic design, energy consumption required by amputees can bedecreased and quality of life improved. Therefore, the ideal prostheticcan be defined in its ability to replace the able-bodied limb in size,shape, and most importantly, functionality. Prosthetic ankles can beevaluated by their ability to mimic the behavior of an intact ankle withregards to kinematics, kinetics, and roll over shape.

In order to mimic the functionality of the ankle during gait, it isnecessary to identify the characteristics of an able-bodied gait thatcannot be achieved without the ankle. Because the SACH foot providesvery little ankle functionality, the gait produced by the SACH foot wascompared to an able-bodied gait to determine the role of the ankle jointduring gait. Data was used from an experimental study looking at theeffect that knee height has on the gait of a transfemoral amputee. Fivesubjects were asked to walk at a self-selected speed for two minutes andfor at least one minute wearing the prosthetic simulator with the SACHfoot shown in FIG. 6.

The gait cycle is used to describe and graph behavior during a typicalstep. The gait cycle begins in stance phase by the heel initiallystriking the ground and exerting a braking force. The beginning 60% ofthe gait cycle is stance phase where the foot is in contact with theground. As the step and stance phase proceeds, the foot becomes furtherdorsiflexed. In dorsiflexion the toes are pointed upward from theneutral position. As stance phase ends, the foot pushes off to propelthe individual forward. During push-off the foot is in plantar flexionwith the toes pointed downward. The gait cycle ends with swing phase torepeat again when the heel is returned to the frontmost position andstrikes the ground. The sagittal plane divides the right and left handsides of the body. Most of the analysis throughout this paper isperformed in the sagittal plane.

The kinematics of the ankle can be described by its angle during thegait cycle. The range of motion of the ankle during gait on an evensurface is no more than 30° and remains consistent with changes inspeed. The ankle angles shown in FIG. 12 supports previous findings.However, the full range of motion of the ankle joint is from about 45°plantar flexion to 20° dorsiflexion. In conclusion, the “ideal”prosthetic ankle should have a range of motion of about 45° plantarflexion to 20° dorsiflexion but exhibit a range of motion of less than30° during gait.

FIGS. 13 and 14 show the ground reaction forces at each subject's normalwalking speed plotted against gait cycle in comparison with anindividual wearing the prosthetic simulator with the SACH foot. Themaximum vertical forces of the able-bodied individuals in FIG. 13exhibit two distinctive peaks that are not seen in the ground reactionforces for the SACH foot. The maximum push-off forces of the SACH footin FIG. 14 are much less than that produced by an able-bodied gait. Somesubjects are able to compensate for the loss of the ankle's contributionto push-off better than others. For example, Subject 4 has the lowestmaximum able-bodied push-off force and the highest maximum SACH footpush-off force. In contrast, Subject 1 has the second highest maximumable-bodied push-off force and the lowest maximum SACH foot push-offforce.

To establish the roll over shape of an able-bodied individual, thecenter of pressure is plotted in a shank-based coordinate system duringstance phase in FIG. 15. Stance phase is established when over 50%percent of each subject's body weight is on the right platform. Theinverted pendulum model approximation and a study of 16 subjectsperformed by Mitchell et al. shows that the ideal roll over shape radiusis approximately 20% body height. As all five subjects were between177-187 cm in height, literature would suggest the best fit radius to be35.4-37.4 cm. The roll over shape of the five able-bodied subjects shownin FIG. 15 has a best fit radius of 38.8 cm and is consistent with thestudy performed by Mitchell et al.

By comparing the roll over shapes of physically impaired and able-bodiedindividuals, characteristics such as a larger radius of curvature (R), alonger arc length, and a longer roll over shape in the X-direction (EFL,Effective Foot Length), have been determined to be preferable.Similarly, a positive x-coordinate center of curvature (X_(c)) is anobservable characteristic of able-bodied roll over shapes and betterprosthetics. S. Miff et al. found that a X_(c) behind the ankle occursduring gait initiation, a X_(c) in front of the ankle occurs during gaittermination, and the X_(c) is at a neutral position during steady stategait. The roll over shape of ankle-foot prostheses that lack adequatepush-off prematurely curve upwards resulting in a smaller best fitradius, arc length, EFL, and center of curvature in the horizontaldirection (X_(c)). The center of curvature in the horizontal direction(X_(c)) of the roll over shape of all five subjects shown in FIG. 15 ispositive as well with a value of 2.02 cm, but smaller than the center ofcurvature for the SACH foot roll over shape (29.3 cm).

While the roll over shape is usually modeled as a circular arc, it hasalso been modeled as a second order polynomial. A second orderpolynomial was found to fit the roll over shape of the CAPA foot betterand used to determine the radius of curvature, X_(c), and forward lengthin the x-direction.

The present invention relates to prosthetic devices. In particular, thepresent invention relates to a compliant and articulating prostheticankle foot.

In one aspect, a prosthetic device includes a phalanges portion, ametatarsals portion that is movably coupled to the phalanges portion, anankle portion that is movably coupled to the metatarsals portion, and acalcaneus portion that is movably coupled to the ankle portion.

In another aspect, a prosthetic device includes a phalanges portion, ametatarsals portion coupled to the phalanges portion, an ankle portioncoupled to the metatarsals portion, and a calcaneus portion coupled tothe ankle portion. At least one biasing member is configured to bias atleast one of the phalanges portion, the metatarsals portion, the ankleportion, and the calcaneus portion in a rotational direction.

In yet another aspect, a prosthetic ankle foot includes an ankleportion, a metatarsals portion, a calcaneus portion, and a phalangesportion. The ankle portion includes a first end with a connector and asecond end with a rocker. The first end is opposite the second end. Themetatarsals portion is rotatably coupled to the ankle portion by a firstbiasing member. The calcaneus portion is rotatably coupled to the ankleportion by a second biasing member. The metatarsals portion and thecalcaneus portion are coupled to the ankle portion on opposite sides ofthe rocker. The phalanges portion is rotatably coupled to themetatarsals portion by a third biasing member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a prosthetic device according to one embodimentof the invention.

FIG. 2 is a perspective view of the prosthetic device of FIG. 1.

FIG. 3 is a top view of the prosthetic device of FIG. 1.

FIG. 4 is a computer assisted rehabilitation environment used to testthe prosthetic device of FIGS. 1-3.

FIG. 5 shows the prosthetic device of FIGS. 1-3 coupled to a prostheticsimulator.

FIG. 6 shows a prosthetic device in the prior art coupled to theprosthetic simulator.

FIG. 7 is a graph that plots the ground reaction forces of theprosthetic device of FIG. 5, the prosthetic device in the prior art ofFIG. 6, and an able-bodied person during gait cycle.

FIG. 8 is a graph that plots the ankle angles of the prosthetic deviceof FIG. 5, the prosthetic device in the prior art of FIG. 6, and rawankle data.

FIG. 9A is a side view of the prosthetic device of FIG. 1, illustratingdifferent positions of the prosthetic device during a gait cycle.

FIG. 9B is a side view of the prosthetic device according to anotherembodiment, illustrating different positions of the prosthetic deviceduring a gait cycle.

FIG. 10A is a perspective view of the prosthetic device of FIG. 1 in afirst position.

FIG. 10B is a perspective view of the prosthetic device of FIG. 9B in afirst position.

FIG. 11 illustrates a dynamic response ankle-foot.

FIG. 12 is a graph that plots the ankle angles of five different testsubjects for an able-bodied ankle and for the prosthetic device in theprior art of FIG. 6.

FIG. 13 is a graph that plots vertical ground reaction forces of fivedifferent test subjects for an able-bodied ankle and for the prostheticdevice in the prior art of FIG. 6.

FIG. 14 is a graph that plots sagittal plane ground reaction forces offive different test subjects for an able-bodied ankle and for theprosthetic device in the prior art of FIG. 6.

FIG. 15 is a graph that plots the roll-over shape of five different testsubjects for an able-bodied ankle and for the prosthetic device in theprior art of FIG. 6.

FIG. 16 is a representation of vector loops on the prosthetic device ofFIG. 5 during a step.

FIG. 17 is a force diagram on the prosthetic device of FIG. 5.

FIG. 18 is a table illustrating fixed geometric parameters used in FIGS.16 and 17.

FIG. 19 is a table illustrating ankle loop equations governing thevector loops of FIG. 16.

FIG. 20 is a table illustrating arm loop equations governing the vectorloops of FIG. 16.

FIG. 21 is a table illustrating effective rotational stiffness valuesfor the prosthetic device of FIG. 1.

FIG. 22 is a table illustrating the effects that individual parametershave on a roll over shape radius of curvature.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

The human ankle allows for rotational movement that resembles a ball andsocket joint and provides the support for ground reaction forces up toten times an individual's body weight. During gait, contraction of theplantar flexors act to create a moment (in N-m) about the ankle jointthat is both twice an individual's body weight and twice the momentcreated about either the knee or hip. Additionally, the forward motionthat occurs during gait is generated primarily by the plantar flexormuscles about the ankle joint. Thus, it is essential for an ankle footprosthetic to mimic the propulsion forces created by the ankle toproduce a natural gait.

With reference to FIGS. 1-3, a prosthetic device (i.e., ankle foot) 10addresses some of the flaws in previous ankle prosthetic systems andbetter mimics a healthy ankle. The prosthetic device 10 may provideadvantages over previous ankle prosthetic systems, such as being lessexpensive, allowing for personalization, and allowing for slopedwalking. The prosthetic device 10 is easily and inexpensively customizedusing 3D printing to better fit individuals of different sizes, naturalgait patterns, and personal preferences. The prosthetic device 10according to one embodiment and illustrated in FIGS. 1-3 comprisesAcrylonitrile Butadiene Styrene (ABS) or Polylactic Acid (PLA) with 100%infill. The device utilizes a rapidly advancing field and models, inadditional or alternative embodiments can be constructed from anysuitable material such as materials that are lighter, more durable, andstronger than ABS or PLA. The visual appeal of the prosthetic device 10can be optimized with 3D printing to avoid the uncanny valley anddevelop a prosthetic that has both a large degree of human likeness andfamiliarity.

With continued reference to FIGS. 1-3, the prosthetic device 10 includesa first or phalanges portion 14, a second or metatarsal portion 18, athird or ankle portion 22, and a fourth or calcaneus portion 26. In oneconstruction, each of the portions of the prosthetic device 10 isprinted from a 3D printer. In the illustrated embodiment, the phalangesportion 14, includes a main body 120 having a first arm 122 extendingfrom the main body 120 at a first end and a second arm 124 extendingfrom the main body 120 at a second end opposite the first end. The mainbody 120 is curved or contoured at is front face and defines a recess126 between the first arm 122 and the second arm 124 to accommodate oneor more biasing members or springs 30. The phalanges portion 14 ismovably (e.g., rotatably) coupled to the metatarsal portion 18 by thespring(s) 30. In the illustrated embodiment, the spring(s) 30 comprisetwo 1.18 N-m 180° steel torsion spring(s), although in other oradditional embodiments fewer or greater spring(s) having differentvalues and materials may be used.

In the illustrated embodiment, the metatarsals portion 18 includes amain body 130 having a first arm 132 extending from the main body 130 ata first end and a second arm 134 extending from the main body 130 at asecond end opposite the first end. The main body 130 also includes athird arm 136 and a fourth arm 138 extending from a face of the mainbody 130, which are received within the recess 126 of the phalangesportion 14. The main body 130 also defines a recess 139 between thefirst arm 132 and the second arm 134 to accommodate one or more biasingmembers or springs 34. The ankle portion 22 is movably (e.g., rotatably)coupled to the metatarsal portion 18 by the spring(s) 34.

In the illustrated embodiment, the ankle portion 22 has a main body 140which includes a rocker 90 (FIGS. 1 and 9A-10B) with a radius that issimilar to the Talus bone in an able-bodied ankle. In anotherembodiment, the rocker 90 includes a radius that is approximately 0.3times a total leg length. In still another embodiment, the rocker 90includes a radius that is approximately 20% of an individual's totalheight. The ankle portion 22 also includes platforms 94 (FIGS. 9A-10B)which define ends of the rocker 90 (i.e., the rocker 90 extends betweenthe platforms 94). The first arm 132 and the second arm 134 of themetatarsals portion 18 are movably (e.g., rotatably) coupled to oneplatform 94.

In the illustrated embodiment, the calcaneus portion 22 includes a mainbody 150 having a first arm 152 extending from the main body 150 at afirst end and a second arm 152 extending from the main body 150 at asecond end opposite the first end. The main body 150 is rounded at isfront face and defines a recess 156 between the first arm 152 and thesecond arm 154 to accommodate one or more biasing members or spring(s)38. The first arm 152 and the second arm 154 of the calcaneus portion 26are movably (e.g., rotatably) coupled to other platform 94 by thespring(s) 38.

In the illustrated embodiment, the springs 34, 38 each comprise 5.0 N-m120° steel torsion springs, although in other or additional embodimentsfewer or greater springs having different values and materials may beused. Each of the springs 30, 34, 38 includes a pair of arms. The armsof the springs 30, 34, 38 are positioned in holes formed in the portions14, 18, 22, 26. The prosthetic device 10 also includes eighth inch(3.175 mm) stainless steel shafts 160, 170, 180 (i.e., joint elements)at each of the movably coupled portions (e.g., shaft 160 extends betweenthe phalanges portion 14 and the metatarsals portion 18, shaft 170extends between the metatarsals portion 18 and the ankle portion 22, andshaft 180 extends between the ankle portion 22 and the calcaneus portion26). The shafts 160, 170, 180 extend through the respective springs 30,34, 38 and for the entire width of the prosthetic device 10. Choices inshaft size and direction of 3D printing were made with tear-out failurein mind.

The device 10 also includes a carbon-fiber and nylon compositepyramid-shaped head 70 (or other suitable connector) that is coupled(i.e., bolted or otherwise secured) onto the ankle portion 22 of theprosthetic device 10. In other constructions, the head 70 may comprisesuitable alternative shapes and is not limited to the particular shapeshown in the figures. The head 70 is attachable to other prostheticpieces or structures (i.e., may be a universal adapter). Finally, theprosthetic device 10 further includes a traction material 80 such asrubber that was painted onto a bottom surface of the prosthetic device10. Any suitable traction material can be used.

The relative motion of the portions 14, 18, 22, 26 allows for theprosthetic device 10 to experience the full range of motion of the anklejoint. Platforms prevent excess flexion for greater stability. Theprosthetic device 10 is classified as a type of dynamic response foot asit stores potential energy at the springs 30, 34, 38 and releases thatenergy to assist in forward movement. Unlike the majority of currentankle systems that only mimic the ESR that occurs in the Achilles tendonfor plantar flexion, the prosthetic device 10 stores energy at eachspring to mimic toe flexion at location 50 in FIG. 1 and both plantarflexion and dorsiflexion at location 60. The springs 30, 34, 38 mimicthe energy storage function of ankle tendons and provide a necessarypush off force for forward motion. The springs 30, 34, 38 can be easilyreplaced with springs of different stiffnesses (not shown) for userswith different walking speeds and body weights.

During the unloading phase of a healthy ankle, there is a linearincrease in the moment exerted by the ankle. This can be emulated by atorsion spring because the force exerted by a spring also follows alinear profile and the angular velocity of an ankle is constant about apoint. The springs can be easily replaced, allowing the same ankle footprosthetic to accommodate different applications or speeds. Eachindividual can adjust the stiffness to what would best reduce theirmetabolic cost of walking. Optimizing the stiffness is important toprovide a balance between the greater propulsive forces provided bystiffer designs and the stabilization stiffer designs require.

In the illustrated embodiment of FIGS. 1-3, the prosthetic device 10 hasa neutral length of 22 cm, 10 cm in width, and 9 cm in height. With aweight between 737.7 g and 887.1 g, the device 10 is heavier than aprosthetic device in the prior art 100 (i.e., the SACH foot, see FIG. 6)that weighs 415.1 g. However, 3D printing the prosthetic device 10 usingdifferent materials such as a carbon-fiber nylon composite can reducethe weight in future models.

The prosthetic device 10 has been designed to create distinctlydifferent roll-over shapes (i.e., a gait characteristic thatincorporates both kinematics and kinetics). Able-bodied individuals mayalter their ankle kinematics in order to maintain their roll-over shape.Amputees, on the other hand, do not have adaptive control over theirroll-over shape. The design of the prosthetic device 10 predominantlycontrols the roll-over shape that the amputee will produce.

The embodiment illustrated in FIG. 9A shows the prosthetic device 10that includes no pretension in the springs 30, 34, 38 in a neutralposition (i.e., between plantar flexion and dorsiflexion where thepyramid head 70 is substantially normal to the ground). The embodimentillustrated in FIG. 9B shows the prosthetic device 10 that includespretension in at least one spring 30, 34, 38 in the neutral position sothat energy is stored in the at least one spring 30, 34, 38 at heelstrike as opposed to when dorsiflexion begins. As shown in FIG. 10B, theplatform 94 of the prosthetic 10 (i.e., where the metatarsal portion 18rests on the ankle portion 22) is angled fifteen degrees lower (i.e., anend of the platform 94 is closer to the ground) in the illustratedembodiment, than in the embodiment illustrated in FIG. 10A. The changein the position of the platform 94 causes the at least one of thesprings 30, 34, 38 to experience a pretension. The heel, the metatarsalportion 18, and the rocker 90 are in contact with the ground duringplantar flexion.

A larger roll-over length (e.g., as measured from a heel to a toe) isfound to be desirable. In the illustrated embodiment, in order toachieve a larger radius within the dimensions of a normal foot, a centerof curvature and a point of contact when the foot is in the neutralposition is moved in from of an ankle marker (i.e., toward the toes).The resulting roll-over shape will also have a center of curvature witha forward shift.

Data was collected using the CAREN 106 (Computer Assisted RehabilitationENvironment) shown in FIG. 4 that is equipped with 10 motion capturecameras, a split-belt treadmill with force plates, 180° of projectionscreens, and a six degree of freedom motion base. The prosthetic device10 was compared to the conventional SACH foot 100 (see FIGS. 5 and 6)using a prosthetic simulator 110 on an able-bodied individual's rightleg. The prosthetic simulator 110 in FIGS. 5 and 6 was assembled from aportion of an iWalk© and a polycentric prosthetic knee. The subject, whoweighed 58 kg, walked at a speed of 0.7 m/s for 1 min first using thesimulator with the prosthetic device 10, then using the simulator withthe SACH foot 100, then walking normally 114. Data from the positioncoordinates from 18 markers and the magnitude and direction of forcesexerted on the treadmill was collected for analysis. Ten steps on theright leg with times within +/−0.3% of the mode step time were chosenand the forces and angles during gait cycle compared.

The braking and push off forces can be analyzed by looking at GRF(ground reaction forces) exerted horizontally in the front to backdirection (z-axis on CAREN 106). FIG. 7 plots ground reaction forceswith respect to gait cycle increasing from heel strike to toe off andstarting when the heel marker is at its front-most position to when itis at its backmost position. At the beginning of the gait cycle, heelstrike is experienced and negative GRF are generated. The step proceedswith push off that produces positive GRF and assists in the forwardmotion of gait. The gait cycle ends in swing phase with close to zeroGRF. FIG. 7 shows that the GRF of the prosthetic device 10 during gaitcycle follows more closely to normal gait than the SACH prosthetic foot100. The average push off force during testing was greater for theprosthetic device 10 (97.7N) compared to the SACH foot 100 (95.9N). Italso can be noted from FIG. 7 that the magnitude of the braking force ofthe prosthetic device 10 and the SACH foot 100 was less than thatexperienced during normal walking.

The ankle angles were computed from the positions of the toe, ankle, andknee markers. FIG. 8 shows that the prosthetic device 10 exhibits asimilar range of motion during gait that an able-bodied individualexperiences, from around 15° plantar flexion to 10° dorsiflexion. Theresults of normal walking were removed from FIG. 8 because the subjectexhibited less dorsiflexion and excessive pronation during gait thatcaused the ankle angles to substantially differ from the well understoodankle angles of an able-bodied individual. Instead, raw ankle angle data118 collected by another source was plotted to demonstrate typical ankleangles. Gait begins with an initial increase in ankle angle for plantarflexion during heel strike and the angle decreases as the step proceedsreaching minimum dorsiflexion just before push off during which plantarflexion occurs. The prosthetic device 10 was shown to emulate the ankleangles of a healthy gait much better than that of the SACH foot 100whose ankle angles remained relatively constant throughout the gaitcycle.

The GRF experienced while wearing the prosthetic device 10 came closerto emulating normal walking than the SACH foot 100. However, the pushoff force was only slightly greater for the prosthetic device 10 despitethe ESR mechanisms of the springs. Stiffer springs could help achieve alarger push off force. Both the prosthetic device 10 and the SACH foot100 fell short of replicating the braking forces during the beginning ofthe gait cycle. However, because the braking force acts against forwardmotion, high braking forces may inhibit an amputee from producing thenecessary forward propulsion from their prosthetic limb. Also, high GRFcould cause greater socket forces and lead to discomfort. With regardsto the movement in the sagittal and transverse planes that a healthyhuman ankle experiences, the design of the prosthetic device 10 fallsshort. Incorporating sagittal and transverse plane movement into thedesign improves stability and walking on uneven terrain. This has beenaccomplished by multi-axial prosthetic ankle foot designs that offer agood alternative to the SACH foot 100 for more active amputees. Futuremodels can integrate some of the beneficial aspects of multi-axialdesigns such as a split foot mechanism to better emulate movement of ahealthy human ankle. Also, shock absorption mechanisms can beimplemented to improve future models.

This experiment demonstrated the potential of the prosthetic device 10to be used by lower limb amputees. When compared to the conventionalSACH foot 100, the ground reaction forces and ankle angles bettermimicked that of a healthy human gait.

In a mathematical model, the prosthetic device 10 (i.e. referred to as“CAPA foot”) may be thought of as a rocker with two arms and a toe inthe 2-dimensional sagittal plane. Using a rotational velocity of theshank and the geometry of the foot at its neutral position, a series ofkinematic equations may be developed to solve for the relative positionsof all components during stance phase. When the components are rotated,potential energy is stored in the springs. This creates a resultantforce at the point of contact between the arm and the ground. The forcedistribution is used to find the center of pressure during the step andis then used to plot the roll over shape.

In the mathematical model, during the beginning of the gait cycle thefoot is in plantar flexion and the heel component is rotated upward. Fora first version of the CAPA foot, only the heel and rocker componentsare in contact with the ground during plantar flexion. For a secondversion, the foot component is in contact with the ground as well. Oncethe shank angle passes the vertical position, the CAPA foot dorsiflexesand only the foot and the rocker is in contact with the ground. The armgeometry is the only difference between the kinematic equationsgoverning the rotation upward of the heel arm versus the foot arm.Therefore, the same kinematic equations can be used. When solving forthe ground reaction forces and force distribution, the stiffness of thejoint is also adjusted according to the spring constant. Thecontribution of the toe is disregarded.

In the mathematical model, and as shown in FIG. 16, points on a rockeror ankle portion 22 and an arm (e.g., a calcaneus portion 26) can beconnected by two loops of vectors. These points and vectors can beconsidered part of either a first rigid body (e.g., a calcaneus portion26) or a second rigid body (e.g., an ankle portion 22). The first rigidbody 26 will rotate about a rotational center 200, and the second rigidbody 22 will rotate about an ankle marker 204 with the rotation velocityof a shank (not shown). Geometrically fixed vector lengths and pointsare shown in black and unknown vector lengths and angles are shown inblue. The fixed lengths and angles are shown in FIG. 18 with the anglesdefined from the positive x-axis.

At every position of the CAPA foot, each of the two vector loops shownin FIG. 16 must make one full circle meaning that each vector sum mustequal to 0. Given the lengths of the vectors when the prosthetic device10 is in the neutral orientation, the vector velocities can be used tosolve for all remaining positions of the vectors. By applying the fixedgeometries listed shown in FIG. 18 and a relationship stating that {dotover (θ)}={dot over (θ)}₁={dot over (θ)}₅, the equations from FIG. 19reduce to the following two equations:

${\overset{.}{r}}_{3} = \frac{{- r_{1}}\overset{.}{\theta}\;{\sin\left( \theta_{1} \right)}}{\cos\left( \theta_{3} \right)}$${\overset{.}{r}}_{4} = {{- r_{1}}\overset{.}{\theta}\;{\cos\left( \theta_{1} \right)}}$The equations from FIG. 20 reduce to the following two equations:

${\overset{.}{\theta}}_{6} = \frac{{r_{5}\overset{.}{\theta}{\cos\left( \theta_{5} \right)}} + {\overset{.}{r}}_{4}}{{- r_{6}}{\sin\left( \theta_{6} \right)}}$${\overset{.}{r}}_{7} = \frac{{r_{5}\overset{.}{\theta}\;{\sin\left( \theta_{5} \right)}} + {r_{6}{\overset{.}{\theta}}_{6}{\sin\left( \theta_{6} \right)}}}{\cos\left( \theta_{7} \right)}$

Given the lengths of the vectors when the foot is in the neutralorientation, the vector velocities can be used to solve for allremaining positions of the vectors. The same parameters are used in theankle loop equations given in FIG. 20 and the equations derived fromFIG. 20 to describe the movement of the ankle portion 22 throughout theentire step. However, different values for r₅, r₆, and θ₇ are useddepending on the arm (the metatarsal portion 18 or the calcaneus portion26) in contact with the ground. For example, when the shank passes thevertical position, the heel arm is not in contact with the groundanymore and there will be no resultant force between the heel arm andthe ground. When the value of r₃ equals zero and the center of curvaturecrosses the ankle marker, the value of θ₃ switches between 0 and 180degrees.

When either of the arms (e.g., the calcaneus portion 26 or a metatarsalsportion 18) is bent upward, biasing members or springs 30, 34, 38 (FIGS.1-3) are compressed at an angle between θ₆. The resultant force F_(arm)(given by F_(arm)=K*({dot over (θ)}₆−{dot over (θ)})) will push againstthe ground at the point of contact between the arm 18, 26 and theground. The remaining forces F_(rocker) will occur at the point ofcontact between the ankle portion 22 and the ground. F_(rocker) isdetermined by subtracting F_(arm) from F_(total), where F_(total) is anexperimentally controlled value. The difference between the x directionlocation of the center of pressure and the ankle marker is given by thefollowing equation:

${x_{lab} - x_{ankle}} = {\frac{- 1}{F_{total}}*\left\lbrack {\left( {F_{rocker}r_{3}{\cos\left( \theta_{3} \right)}} \right) + \left( {F_{rocker}r_{7}{\cos\left( \theta_{7} \right)}} \right)_{heel} + \left( {F_{rocker}r_{7}{\cos\left( \theta_{7} \right)}} \right)_{foot}} \right\rbrack}$where θ₃ or θ₇ are 0 or 180 degrees. These points can then be plotted toform the roll over shape.

The quasi-stiffness of the human ankle can be evaluated by measuring theslope of the ankle angle versus ankle moment graph. An alternative wayof determining the joint stiffness required by the CAPA foot is to lookat the discrepancy between the gait of an able-bodied individual and thegait of the same individual wearing the SACH foot that provides verylittle push-off. FIG. 14 shows the discrepancy to be approximately 10%the individual's body weight. The average participant in the studyweighed 72.22 kg so the CAPA foot must reach 70.8N of force at 10degrees dorsiflexion. Therefore, a rotational stiffness of

$7.08\frac{N}{\deg}$was used to guide the effective rotational stiffness values given inFIG. 21.

As shown in FIG. 22, after determining the spring constant, thegeometries of the CAPA foot can be chosen to produce the desired rollover shape (e.g., ability to personalize to a specific roll over shape).Additionally, increasing the stiffness at either the heel or toelengthens the roll over shape. Increasing the distance r₁ between theankle marker and the center of curvature of the ankle 22 by using alarger radius will cause the point of contact between the ankle 22 andthe ground to move more during the step, which will result in a flatterand longer roll over shape. A length of the arm piece (e.g., thecalcaneus portion 26) may also be increased. Increasing the length r₆ ofthe arm piece 18, 26 will increase a distance of a point of contactbetween the arm and the ground and the ankle marker causing the centerof pressure to move further forward and also causing the roll over shapeto lengthen.

Although the invention has been described in detail with reference tocertain preferred embodiments, variations and modifications exist withinthe scope and spirit of one or more independent aspects of the inventionas described.

What is claimed is:
 1. A prosthetic device comprising: a phalangesportion; a metatarsals portion that is movably coupled to the phalangesportion at a first connection point; an ankle portion that is movablycoupled to the metatarsals portion at a second connection point spacedapart from the first connection point; and a calcaneus portion that ismovably coupled to the ankle portion at a third connection point spacedapart from the first and second connection points; wherein at least onebiasing member rotatably couples the metatarsals portion to thephalanges portion, at least one biasing member rotatably couples themetatarsals portion to the ankle portion, and at least one biasingmember rotatably couples the calcaneus portion to the ankle portion;wherein each of the at least one biasing members is a torsion spring. 2.The prosthetic device of claim 1 wherein the ankle portion is configuredto be coupled to another prosthetic structure.
 3. The prosthetic deviceof claim 1 wherein the first connection point includes a first recess,the second connection point includes a second recess, and the thirdconnection point includes a third recesses, and wherein the firstbiasing member is disposed within the first recess, the second biasingmember is disposed within the second recess, and the third biasingmember is disposed within the third recess.
 4. The prosthetic device ofclaim 1 wherein at least one of the phalanges portion, the metatarsalsportion, or the calcaneus portion is a flat plate.
 5. A prostheticdevice comprising: a phalanges portion; a metatarsals portion coupled tothe phalanges portion at a first connection point; an ankle portioncoupled to the metatarsals portion at a second connection point spacedapart from the first connection point; a calcaneus portion coupled tothe ankle portion at a third connection point spaced apart from thefirst and second connection points; and at least one biasing memberconfigured to bias at least one of the phalanges portion, themetatarsals portion, the ankle portion, or the calcaneus portion in arotational direction; wherein the at least one biasing member includes afirst biasing member that rotatably couples the metatarsals portion tothe phalanges portion, a second biasing member that rotatably couplesthe metatarsals portion to the ankle portion, and a third biasing memberthat rotatably couples the calcaneus portion to the ankle portion;wherein the at least one biasing member is a torsion spring.
 6. Theprosthetic device of claim 5 wherein the ankle portion includes arounded portion extending between the metatarsals portion and thecalcaneus portion.
 7. A prosthetic ankle foot comprising: an ankleportion including a first end with a connector and a second end with arocker having a curved surface configured to contact the ground, thefirst end opposite the second end; a metatarsals portion rotatablycoupled to the ankle portion by a first biasing member; a calcaneusportion rotatably coupled to the ankle portion by a second biasingmember, the metatarsals portion and the calcaneus portion coupled to theankle portion on opposite sides of the rocker; and a phalanges portionrotatably coupled to the metatarsals portion by a third biasing member;wherein each of the first, second, and third biasing members is atorsion spring.
 8. The prosthetic device of claim 7 wherein theconnector is configured to be coupled to another prosthetic device. 9.The prosthetic device of claim 7 wherein at least one of the first,second, and third biasing members is configured to be under pretensionwhile the connector is substantially normal to the ground.